Actually this is CollisionFree. TiddlyWiki uses "--" to mean mdash and also uses --strike-- formatting. The formatter distinguishes between a freestanding "--" and "--" wrapping a word or phrase. This is fairly straightforward to implement (I know, because I implemented it for TiddlyWiki).

Not everyone puts spaces around dashes and signatures -- it is the current recommendation for modern typography, but some people follow old style or like to experiment. Also note how this looks in the raw page text -- as if someone inteded to insert a word surrounded by dashes, just didn't make the spaces. But this is a minor issue, and indeed there is a million ways to solve that.

The fundamental issue is "Do we need it hard enough to make Creole more complicated"?

Another concern about it being CollissionFree -- looking at the markup summaries on Wikimatrix I see that engines use "--" and "~" interchangeably. So, no matter which one we choose for "deleted text", we will introduce a conflict with some of the existing markups.
-- RadomirDopieralski, 2006-12-18